ROJECT SUMMARY (See instructions): We have found that mice that are unable to generate Thi7 cells and mice that are unable to respond to IL- 17 (due to global loss of IL-17 receptor C) are protected from airway hyperresponisveness in two models of allergic asthma. We also found that IL-17A directly increases the contractility of both murine and human airway smooth muscle through a pathway we characterized involving activation of NF-kB, increased expression of RhoA and enhancement of myosin phosphorylation. In addition, we found dramatic synergy between IL-17 and the Th2 cytokine IL-13 in enhancing airway smooth muscle contraction. In the proposed studies we will first determine the relative contributions of direct effects of IL-17 on airway smooth muscle or adjacent airway epithelium in vivo utilizing mice that specifically lack IL-17RC or the key IL-17 receptor signaling molecule, Act-1, only in smooth, only in lung epithelial cells or in both cell types, and by analyzing effects on contraction of human bronchi with or without the overlying epithelium. Because synergistic interactions between IL-17 and IL-13 might have special relevance to therapeutic strategies in asthma, we will also carefully disect the site(s) of signal amplification that underlie the synergistic effects of these cytokines on human airway smooth muscle, including smooth muscle cells cultured from patients with asthma. Finally, we will utilize the 3 cohorts of patients with asthma to be analyzed in the Clinical Subject and Biospecimen Core to determine the relationships between the signaling responses in airway smooth muscle cells, the number and location of IL-13 and IL-17 producing cells and airway hyperresponsiveness in patients with moderate or severe asthma and in responses to segmental allergen challenge. These studies should complement the studies proposed in Project 1 to study effects of the same cytokines on airway epithelial cells and the studies proposed in Project 3 to examine the dynamics of IL-13 and IL-17 production and signaling in asthmatic airways. Together, the proposed studies should provide important details about how these critical cytokines contribute to the initiation and persistence of allergic asthma and should identify novel steps for intervention.